Carbon--carbon composites are used for rocket nozzle throats and exit cones, aircraft brakes, hypersonic vehicle leading edges and other high temperature structures. Current carbon--carbon densification processes are costly and slow, involving either 6-8 pitch impregnation-carbonization cycles or 400-600 hours of chemical vapor infiltration (CVI). It is often necessary to interrupt the CVI process and machine off the exterior surfaces of the substrate to open pore channels to the interior. Current CVI processes cannot densify low-permeability preforms or parts thicker than about 11/2 inches. The present inventive method reduces process time and improves densification.
Carbon--carbon (C--C) composites CVI densified with pyrolytic carbon matrices are an erosion resistant and structurally efficient material used on solid rocket motor nozzle exit cones and C--C brake disks for commercial and military aircraft. The current process, known as isothermal CVI, is costly and limited to thin components. In the isothermal CVI process the gas, usually methane, flows past parts resting in a furnace. Gas transport into the interior of the part is driven by diffusion. To deposit uniformly through the thickness and prevent closing off surface pores, the isothermal process uses extremely low deposition rates, requiring hundreds of hours of deposition. Even with very low rates, uniform densification of most substrates is difficult in preforms thicker than 50 mm (2.0 inch). Isothermal CVI densification is, therefore, limited to components such break discs and as the thin nozzle exit cone. The high cost further limits the material to the nozzles of strategic and space motors only. To densify thicker parts such as the nozzle integral throat entry (ITE), a pressure impregnation-carbonization process is used. As many as eight impregnation-carbonization and graphitization cycles are required. Lower cost carbon--carbon ITEs are needed for future propulsion systems. Other existing and potential applications such as spacecraft structures, automotive pistons and aircraft brakes would also benefit from improvements in densification processes.
The following references further describe the field of art and are herein incorporated by reference:
U.S. Pat. No. 4,134,360, Apparatus for Vapor Deposition on Tubular Substrate, discloses a process utilizing radial forced flow through a thick walled cylinder and a solid log preform. The part is directly heated by inductive coupling of the part. This process can process only one stack or log at a time in a furnace and the furnace must be cooled after each load. There is no active internal cooling. Thus, unless longer densification processes are used or thinner substrates used, preferential deposition of the inside surface will result and the part will be unacceptable for desired applications. Greater pressure must be applied to permit densification of individual disks using hydraulic or air actuated ram, which is not required in the present inventive process.
U.S. Pat. No. 4,212,906, Method for the Production of Carbon/Carbon Composite Material, which is a continuation of U.S. Pat. No. 4,134,360, discloses densification of a long continuous substrate which is subsequently parted into individual brake disks.
U.S. Pat. No. 4,580,524, Process for the Preparation of Fiber-Reinforced Ceramic Composites by Chemical Vapor Deposition, is specific to ceramic composites and employs steep thermal gradient. The deposition is progressive, where the process gas first preferentially reacts at and deposits on the fibers in the region of the hot surface. Deposition occurs progressively from the hot surface toward the cold surface and edges. In contrast, the present inventive process produces a substantially uniform deposition rate throughout the substrate. Unlike the substrate of the process of the present application, the substrate in '524 must be thin, due to the deposition progression from the outer to inner surface. Further, the '524 process deposits ceramic matrix, specifically SiC, S.sub.3 N.sub.4.
U.S. Pat. No. 4,895,108, CVD Preparation of Fiber Reinforced Ceramic Composites, discloses a similar process and apparatus to the process for densification of cylindrical substrates of U.S. Pat. No. 4,580,524 (Lackey), but indicates cooling of the inner surface with the Lackey process is inadequate. Enhanced cooling of the inner surface is achieved by a water cooled mandrel in contact with the part inner surface. The mandrel has circumferential and axial grooves on its surface for gas distribution. Water flows through concentric tubes at the core centerline.
U.S. Pat. No. 5,350,545, Method for Fabricating Composites, discloses rigidization using preceramic polymers prior to CVI densification. The thickness of the substrate is limited to a few centimeters and the process utilizes a ceramic matrix only.
U.S. Pat. No. 5,348,774, Method of Rapidly Densifying a Porous Structure, discloses a process utilizing a thermal gradient only, no forced flow. The part must be heated by inductive coupling of the part with an RF field from the furnace coil.
U.S. Pat. No. 5,480,678, Apparatus for Use With CVI/CVD Processes, discloses a forced flow process without a thermal gradient. The process is specific to stacks of thin parts separated by spacers which direct the flow of the process gas through the parts.
The following are herein incorporated by reference:
Fiber Reinforced Tubular Composites by Chemical Vapor Infiltration, D. Stinton, et al. Chemical vapor deposition of refractory metals and Ceramics ll, Proceedings of the symposium, Boston, Mass., Dec. 4-6, 1991, Pittsburgh, Pa., Materials Research Society. The process uses a water cooled tube to cool inside surface of specimen.
Deposition Kinetics in Forced Flow/thermal Gradient Chemical Vapor Infiltration, T. Starr, Georgia Institute of Technology, 12th Annual Conference on Composites and Advanced Ceramic Materials, Cocoa Beach, Fla., Jan. 17-22, 1988, Ceramic Engineering and Science Proceedings, Vol. 90, 1988, pp. 803-811. Kinetic modeling of Ceramic FCVI.
Synthesis of Fiber-reinforced Sic Composites by Chemical Vapor Infiltration, D. Stinton, et al, American Ceramic Society Bulletin, Vol. 65, February 1986, pp. 347-350.
Fabrication of Ceramic Composites: Forced CVI, T. Besmann, presented at 96th Annual meeting of the American Ceramic Society, Indianapolis, Ind., Apr. 24-28, 1994.
Scale-up and Modeling of Forced Chemical Vapor Infiltration, T. Besmann, 18th Annual Conference on Composites and Advanced Ceramic Materials, Cocoa Beach, Fla., Jan. 9-14, 1994, Ceramic Engineering and Science Proceedings.
Modeling of Chemical Vapor Infiltration for Composite Fabrication, T. Starr (GTRI), T. Besmann (ORNL), presented at International Conference on High Temperature Ceramic Matrix Composites, Bordeaux, France, Sep. 20-25, 1993.
Modeling of Forced CVI for Tube Fabrication, T. Starr (GTRI), A. Smith (ORNL), ORNL Report No. DE94-004544.
Finite Volume Model for Forced Flow/thermal Gradient Chemical Vapor Infiltration, T. Starr (GTRI), A. Smith (ORNL), ORNL Report No. DE91-012531.
Development of High-density Silicon-carbide FCVI Composites, Y. Roman, et al, ORNL Report No. DE91-000922.
Fabrication of Carbon--carbon Composites by Forced Flow Thermal Gradient Chemical Vapor Infiltration, S. Vaidyaraman, J. Lackey, et al., Journal of Materials Research, Vol. 10, No. 6, June 1995, pp. 1469-1477.
Review, Status and Future of the Chemical Vapor Infiltration Process for Fabrication of Fiber-reinforced Ceramic Composites, W. Lackey, 13th Annual Conference on Composites and Advanced Ceramic Materials, Cocoa Beach, Fla., Jan. 15-18, 1989, Ceramic Engineering and Science Proceedings, Vol. 10, 1989, pp. 577-584.